System for reproducing pulse time modulated waveforms stored along a diffractive track

ABSTRACT

A record system for reproducing information recorded on a record. Information is recorded on a track of the record in the form of an embossed print made of diffractive elements representing the time variation of a pulse modulated waveform. The diffractive elements have a width on the order of the wavelength of a readout light beam. An optical beam readout device provides the concentrated read-out beam and senses a spatial variation in the diffractive energy emerging from an illuminated portion of the track.

This is a continuation of application Ser. No. 108,499 filed Dec. 31,1979, abandoned, which is a continuation of Ser. No. 793,270 filed May3, 1977, abandoned, which is a continuation of Ser. No. 390,715, filedAug. 23, 1973.

The present invention relates to a system for reproducing from a recorddata-carrying signals stored upon a substrate the surface of which,along a rectilinear or curvelinear track, exhibits irregularities in theform of depressions or projections, which correspond with the timevariation in the signals carrying said data. This time variation isoptically detected during the scanning of the track, by the effectexercised by these surface irregularities upon the optical diffractionof a concentrated read-out beam which converges onto the engravedsurface of the record.

The invention likewise relates to a record or data-carrier appropriateto said method of recording by virtue of depressions or projections, andto a reproducing device which makes it possible to effect opticalread-out of the signal stored in relief form at the surface of saidcarrier.

In the known methods of reproducing signals which are stored in the formof superficial deformations of the surface of a record, for read-outpurposes, use is made of a stylus which displaces in a groove impartingto the stylus lateral or depth displacements. The movements imparted bythe modulation in the groove or track, to the tip of the stylus, arecommunicated to an electromechanical transducer which supplies a voltageproportional to the instantaneous velocity or amplitude of the tip ofthe read-out stylus. These methods have numerous drawbacks, the major ofwhich are the wear in the stylus, in the carrier or substrate, the playback distortion and the difficulty of making the stylus assembly of thetransducer follow the high frequency components which succeed oneanother at very close intervals.

Also known are methods of optically reading out a carrier carrying aphotographically recorded track and exhibiting variations in opticaldensity or in width, corresponding to the time variation of a signal. Adifficulty with these systems, resides in the fact that the copies ofthe carrier are obtained by photochemical methods, more expensive thanthe die-stamping methods employed with ordinary sound record discs.Another difficulty of these optical methods which are based upon theprojection of enlarged images, resides in the small depth of field ofthe objective lenses used to form the enlarged image of the track, andin the limitation imposed upon the resolving power, due to thephenomenon of diffraction. In order to overcome these difficulties, ithas been proposed a method of holographic recording by which the carriermay be copied by a stamping operation, but this technique involves theuse of relatively powerful coherent light sources since the diffractionefficiency of the recorded holograms is relatively poor. Hitherto, themajor difficulty with systems for mechanically or optically scanning thecarriers on which the signal is recorded, has consisted in maintaining aproportional relationship between the amplitude of the deformationrecorded on the track, and the electrical signal furnished by theread-out equipment. This amplitude proportionality can be roughlysatisfied if the stored signals are pulse time modulated waveformstaking the form of successive square wave envelopes containingtransitions between two levels. The precise location of such transitionsin time, serves to faithfully translate the information. This isparticularly the case in telecommunications systems utilising frequencyor phase modulation. There is nothing to prevent this principle frombeing applied to the field of optical recording with a view tosimplifying the process of scanning and reading out a record wherein,the embossed pattern engraved corresponds to a purely time variation onthe part of an alternating data-carrying signal.

The present invention relates to a material data-carrier or recorddesigned to be read out optically by transmission or by reflection, atleast one of the engraved faces of which comprises, arranged inaccordance with at least one recorded track, a series of irregularitiesin the form of depressions or projections, corresponding to the timevariation of an alternating signal; one level of said alternating signalis translated by a portion of said track not containing an irregularity,another level of said alternating signal is translated by a portion ofsaid track which does not contain an irregularity, the width of which,measured perpendicularly to the track axis, does not exceed two micronsand the length of which, along said track, is equal to or greater thansaid width. Microscopic examination of the surface of the record, wouldexhibit the presence of a chain of diffractive elements of more or lesselongated form and, in accordance with an essential feature of theinvention, it is purely by virtue of their diffractive power, that thedepressions or projections at the surface of the carrier, serve toreconstitute the recorded alternating signal. Their shape is in no waycritical and they can be readily obtained for example by chemicallyetching a smooth plate through a suitably exposed photosensitive resinmask. The optical accuracy and fidelity of the transcription of thesignal, depend upon the location, upon the track, of the diffractiveelements which constitute it, and not upon particularly careful cuttingof its surface or upon the change in its optical properties, in the waywhich is the case in the known systems. The small transverse dimensionsof the track, due to the systematic exploitation of the phenomenon ofdiffraction, mean that an extremely high data-storage capacity isachieved. If the record takes the form of a disc and if the track is ofspiral shape, then the neighbouring turns of the spiral can be locatedvery close together indeed; the result is that, in accordance withanother feature of the invention, read-out is performed using aconcentrated light spot having a diameter comparable with the width ofthe track; the centring of this light spot to within a fraction of amicron, is achieved by positional control of the light sourceilluminating the track, during the read-out phase.

The invention likewise relative to an optical read out device forreproducing the engraved record comprising illuminating means thewavelength of which is of the same order of magnitude as the width ofthe track; these means, at the surface of the carrier, form a light spotof diameter substantially equal to the track width; the centering ofthis spot is ensured, as required, by the use of positioned controlunder the command of two photodetectors picking up the light diffractedby the track.

In accordance with the present invention there is provided a system forreproducing from a recorded track having an axis, a pulse time modulatedwaveform, said system comprising: a record having at least one engravedface carrying an embossed print of said recorded track, and associatedwith said record a reproducing device arranged for optically reading outsaid embossed print; said embossed print being constituted by asuccession of diffractive elements distributed along said axis inaccordance with the sequence of pulses of said waveform; the width ofsaid embossed print being substantially constant along said axis, andthe non-uniform length and spacing of said diffractive elements being atleast equal to said width; said reproducing device includingillumination means for projecting a concentrated spot of radiant energyonto said embossed print, photo-electric detection means arranged forselectively collecting the diffracted radiant energy emerging from theportion of said embossed print illuminated by said concentrated spot,and moving means arranged for following with said concentrated spot, thepath of said embossed print.

For a better understanding of the present invention, and to show how thesame may be carried into effect, reference will be made to the ensuingdescription and the attached figures, among which:

FIG. 1 is an isometric view illustrating the record in accordance withthe invention, and its optical read-out device.

FIG. 2 is a partial isometric view illustrating a fragment of the recordshown in FIG. 1, at substantial magnification.

FIG. 3 is a sectional view illustrating the essential elements of aread-out device centred in relation to the track being read out.

FIG. 4 is a sectional view illustrating the device shown in FIG. 3 in aneccentric position.

FIG. 5 is a sectional view of a first variant embodiment of the read-outdevice shown in FIG. 3.

FIG. 6 is a sectional view of a second variant embodiment of theread-out device shown in FIG. 3.

FIG. 1 illustrates a record 1 in the form of a circular disc which canrotate in its own plane, at an axis 4, thanks to the provision of adrive pin 2 mechanically connected to a motor 3. The bottom face of thedisc 1, parallel to the plane x o y, is assumed to be smooth, and thetop face 16, parallel to the latter is also smooth but contains asuccession of diffractive elements 14 in the form of depressions orprojections, arranged in the form of the turns 15 of a spiral track.Each of the elements 14 has a closed contour, in the plane of the face16, of more or less elongated shape, the width 1 of which issubstantially constant and does not exceed two microns. The element 14can take the form of a shallow trough hollowed out of the surface of theface 16, or of a bead. These various relief irregularities, are producedfor example by chemical etching circumscribing the contours of thesesuperficial irregularities; the areas subjected to this etching, aredelimited by a masking technique which involves selective exposure of aphotosensitive resin. It is also possible to use an imprint pressed by adie, in the manner employed in the manufacture of micro-circuits. Theimportant thing to bear in mind as far as the manufacture of the disc isconcerned, is that the exposure of the resin used for the maskingfunction, is effected by a light spot the intensity of which ismodulated by a pulse time modulated squarewave electrical signal. Theincorporation of the data into the modulating signal is carried out, asrequired, by frequency of phase-modulation or by any other coding methodcapable of producing a pulse-coded message.

To conclude the description of the make-up of the record 1, in FIG. 1,it has been assumed that the disc is transparent so that it can be readout by transmission. It has also been assumed that it is rigid althoughit is equally possible to envisage utilisation of a flexible disc whichslides between stabiliser shoes which have not been shown.Self-evidently, the shape of the record 1 is in no way limitative; arecord of tape form, containing one or more rectilinear tracks, isconceivable, and in the case of a circular disc, instead of the spiraltrack a set of concentric circular tracks could be substituted, givingstep-by-step access to the recorded data.

In addition to the record 1 and its drive system, FIG. 1 also shows theoptical read-out device employed in relation to the track 15. Thisread-out device is essentially constituted by a light source 5 and anobjective lens 7. The source 5, parallel to the axis oz, producessubstantially parallel light beams 6 and the microscope objective lens 7causes the beam 6 to converge at the point O on the track 15. The lightrays 9 which converge towards the point O, intersect and diverge beyondsaid point; after having passed through the disc 1, a fragment of whichhas been removed in order to simplify the drawing, they illuminate anarea 10 which overlaps to a greater or lesser extent the receivingsurfaces of two side-by-side photodetector elements 12 and 13. The spaceseparating the receiving surfaces of the photodetector elements 12 and13, is located plumb in line with the direction oz and orientated alongthe axis of the track, tangentially to ox at the read-out point O.

The photodetector elements 12 and 13 furnish electrical signals whichare applied respectively to the inputs of a first differential amplifier17. The output of the amplifier 17 is connected to a low-pass filter 21.This filter 21 supplies an error voltage ε which, through the medium ofan electromechanical transducer 8, controls the radial displacement inthe o y direction, of the objective lens 7. The electrical signalsfurnished by the photodetectors 12 and 13 are also applied to resistors19, which, with the resistor 20 and the operational amplifier 18,constitute an electrical transmission circuit furnishing a signal S(t)proportional to the sum of the signals produced by the twophotodetectors 12 and 13.

When the point of convergence O of the beam 9 encounters the surface 16between two diffractive elements 14 succeeding one another on the track15, no diffraction occurs and the light energy received by thephotodetector elements 12 and 13 is confined to the interior of the area10.

By contrast, as soon as the point of convergency O of the beamencounters a diffractive element 14 on the record 1, the lightexperiences substantial diffraction, this tending to distribute thelight energy over a cross-hatched area 11 which substantially exceedsthe area 10. The result is a variation in the sum S (t) of the signalsfurnished by the two photodetector elements 12 and 13. At the time ofpassage of the elements 14, there is picked up at the output of theamplifier 18 a signal S(t) of squarewave form, which faithfullytranslates the time variations in the signal engraved in the track 15.As far as the signal S(t) is concerned, it is not mandatory to providetwo photodetector elements 12 and 13. The sensitive faces of these twotransducers can be combined into one, and an optical mask provided tocover the area 10. Under these conditions, overlapping of the area 10which is masked, will indicate the passage of a diffractive element 14,in the form of an appreciable variation in the voltage produced by thesingle photodetector. Instead of providing a mask, it is simpler toseparate the photodetector into two parts, as shown in FIG. 1. Thisdoubling-up has the additional advantage of making it possible to detectdeviations on the part of the point of convergence O in relation to theaxis of the track 15.

FIG. 2, at a very large magnification, illustrates a tiny fragment ofthe record 1, located at the position of the read-out device. Toillustrate the shape of the depressions and projections which theengraved profile can exhibit, on a turn 15, surface irregularities 14 inthe form of more or less elongated troughs have been shown; on theneighbouring turn 150, the irregularities 141, 142 and 143 areconstituted by more or less elongated beads. The read-out beam 9 hasalso been illustrated by showing its contour. The objective lens 7,because of its imperfections and because of the diffraction of thelight, forms at the surface 16 of the record 1 a spot 22 whichsubstantially occupies the width 1 of the track 15. This spot has aminimum diameter of the same order of magnitude as the wavelength λseparating two successive wve surfaces Σ of the read-out radiation 9.The envelope of the illuminating beam has its minimum section at thefocus O of the objective lens 7 and it flares only gently around thisposition so that the record system is able to make verticaldisplacements of several microns without any substantial change in theconditions of illumination of the track.

To illustrate the foregoing, in FIG. 2 there has been produced inrelation to the projecting elements 141, 142 and 143 of the turn 150, adiagram which represents the time variation of the signal S recorded inthis section of the turn. The signal S is a pulse time modulatedsquarewave signal, the top levels AB, C and DE of which correspond tothe scanning of the beads 141, 142 and 143 by the beam 9. During thescanning of the turn 150, the carrier 1 moves at the speed V=ωR, where ωis the angular velocity of the disc and R the radius of the turn. Thebottom levels of the signals S, correspond to the transitions away fromthe parts of the turn. The pitch p of the turns is chosen so that thelight spot 22 can only read one turn at a time. The pitch p will be forexample two to three times greater than the width 1 of the turns, sothat clearance bands of smooth reference surface 16 flank the tracks 15.

So far as the distribution of the light energy is concerned, from theenlarged view of FIG. 2 it will be seen that the light rays 9 emergingfrom the objective lens 7 cross one another in a pseudo-conical volumewhich has its minimum section 22 at the level of the focus O. In theabsence of diffraction at the surface 16, the light energy is confinedto this volume. By contrast, if the beam 9 is intercepted by adiffracting element 14, dispersion of the light energy within the solidangle having its apex at the point O takes place. A photodetectorlocated in the volume delimited by the dotted lines flaring beneath thelight spot 22 will experience a variation in the luminous intensityreceived in the presence of diffraction. The same photodetector couldequally well be arranged outside this volume. In addition, if the lightvolume forming at the exit from the lens 7 is split into two parts andif these parts are located to the left and to the right of the plane xozcontaining the axis of the track and the axis of the beam, thensymmetrical distribution of the light energy will be obtained when thelight spot 22 is centered on a diffractive element 14 and the track 15.

If the light spot 22 is eccentric by the quantity Δy in relation to theaxis of the track 15, the light energy fractions will be distributedasymmetrically to either side of the plane xoz; it is thus possible todetect the offset Δy by utilising two photodetector elements.

The sectional view of FIG. 3 corresponds to the plane of the section yozof FIG. 1, and illustrates the path taken by the light rays when thelight beam 9 concentrated by the objective lens 7 is centered on adiffractive element 14. In the absence of any diffraction, the lightenergy is confined between the rays 26 illustrated in dotted line, and asmall portion of this light energy reaches the receiving surfaces of thephotodetector elements 12 and 13 which are separated by the distance S.In this case, equal currents S₁ and S₂ are delivered by thephotodetector elements 12 and 13 and if we consider the circuits of FIG.1, it will be seen that the amplifier 17 produces a zero voltage, thisbeing translated into terms of an error signal ε which produces nodisplacement of the objective lens 7 under the influence of theelectromechanical transducer 8.

When a diffractive element enters the path of the light the marginalrays 25 appear and define, with the rays 26, the illumination zones 27and 28 which cause the currents S₁ and S₂ to vary by the same quantity.The sum of these variations is available at the output of the amplifier18, in the form of a signal S (t) which represents the scanning of theelement 14 by a change in level. By contrast, because of the quality ofthese variations, the voltage produced by the differential amplifier 17remains unmodified.

In FIG. 4, the path of the light rays has been illustrated in the caseof an offset Δy on the part of the axis of the objective lens 7, inrelation to the track element 14 being read out. It will be seen thatthe zone 27 contains a different energy fraction to the zone 28, becauseof the offset; the result is variations in the currents S₁ and S₂ and inthe error voltage ε produced by the filter 20, this voltage acquiring avalue which, under the action of the transducer, tends to return theobjective lens 7 to the center of the track being read. The positionalcontrol or feed-back thus achieved through the agency of the transducer8, makes it possible to maintain the read-out beam in a perfectlycentred relationship in the case of radial fluctuations of the trackbeing read, of as much as several tens of microns; the low-pass filterserves to eliminate the high-frequency components which could disturbthe operation of the control.

The control of the objective lens 7 makes it possible to correctlyfollow a small-pitch spiral trace, despite eccentricity errors in thedisc. In the event that it looses the track, the system tends to lockonto an adjacent track, but this could take place without being detectedin the case of recording a television video signal, because it ispossible to arrange for each revolution of the track to contain thevideo signal of a television frame. In this case, the track jump will beunobserved because the synchronism of operation is undisturbed.

FIG. 5 illustrates a practical example of the system in accordance withthe invention. This example is in no way limitative of the scope of theinvention and relates to the case of a transparent disc engraved on oneface. The beam 6 is produced by a helium-neon laser operating at a powerof 1 milliwatt. The wavelength of emission is equal to 0.6328 micronsmaking it possible to choose a diffractive track having a width 1 equalto 1 micron. The diffractive elements 14 take the form of more or lesselongated troughs with a depth of 0.5 microns. The two photodetectorelements 12 and 13 are silicon cells whose sensitive surfaces have areasof 2 mm² and are separated from one another by a distance s equal to 0.8mm. The sensitive surfaces are located at around 6 mm from the surface16 of the disc 1 and are fully illuminated by the light beam coming froma microscope objective lens 7 having a magnification of 80. Theelectromechanical transducer 8 which is used to radially displace theobjective lens 7, is constituted by a ceramic electrostrictive bimorphdevice; it is excited by a low-pass filter 21 whose cut-off frequency issome few hundreds of cycles per second.

In the variant embodiment described hereinbefore, the read-out of therecord is effected by transmission so that only one face of the disc canbe engraved. In order to double the storage capacity of a disc, it canbe engraved on both faces provided that the track is read out byreflection.

In FIG. 6, a record 1 has been illustrated the faces 16 and 160 of whichcan be engraved in a similar fashion to that illustrated in FIGS. 1 and2. The record 1 can be made or a non-transparent material and itsengraved faces can, if required, be given a thin reflective coating. Theread-out head comprises the same elements as the one hereinbeforedescribed, but the beam 6 is hereby transmitted to the objective lens 7through a semi-reflective plate 100; this serves to reflect the light ina reverse direction which it takes through the objective lens 7, towardsthe photodetector elements 12 and 13 arranged laterally and above thesurface 16. In the absence of any diffractive element 14 in theilluminated zone of the surface 16, a reflected beam similar to that 6is received by the photodetector elements 12 and 13. By contrast, in thepresence of a diffractive element, the reflected light is diffracted inthe directions 25 outside the pupil of the objective lens 7; the resultis a reduction in the luminous intensity incident upon the photodetectorelements 12 and 13. Apart from these modifications, the mode ofoperation is essentially the same as that of the read-out devicesoperating by transmission.

Utilisation of the system in accordance with the invention makes itpossible to record a band of frequencies reaching the order ofvideo-frequency signals, upon a disc-type record to which the techniqueof reproduction by hot-pressing a thermoplastic material, can beapplied. A disc of this kind can be enclosed in a dustproof cassette andequipped with a radial window to enable it to be optically read out on arecord player of light-scanner type, without it being necessary toutilise a coherent light source. Self-evidently, read-out is not limitedto the visible spectrum and it is merely necessary to use a read-outradiation which corresponds to the sources and photodetectors employed.

What I claim is:
 1. A system for reproducing from a recorded trackhaving an axis, a pulse time modulated waveform, comprising:a recordcontaining said recorded track and having at least one referencesurface, said recorded track being formed from non-contiguous surfaceirregularities; uniform upward and downward facing media abutting onsaid reference surface for forming a modulating structure; the edges ofsaid surface irregularities having closed contours in said referencesurface, the width of said closed contours measured transversely of saidaxis being substantially constant and not in excess of two microns; thenon-uniform length and spacing of said closed contours along said trackbeing at least equal to said width; said surface irregularities formingin said reference surface a set of adjacent track portions; saidadjacent track portions being flanked by clearance bands consisting ofsmooth portions of said reference surface; a reproducing device arrangedfor optically reading out said modulating structure; said reproducingdevice including illumination means for projecting a spot of radiantenergy converging onto said phase modulating structure, said spotsubstantially occupying the width of any one of said track portions,light emerging from said track portion having a spatial distributionrepresenting an amount of track misregistration between said spot andtrack, photoelectric detection means arranged for partially collectingthe modulated radiant energy emerging from the portion of saidmodulating structure illuminated by said spot, and detecting an amountand direction of track misregistration from said spatial distributionand positional control means for causing said spot to displace withrespect to the read-out track to compensate for any misregistration ofthe spot and track; said photoelectric detection means comprising atleast one photodetector assembly for sensing the spatial distribution ofsaid modulated radiant energy, said photodetector assembly beingpositioned in a detection plane.
 2. A system as claimed in claim 1,wherein said surface irregularities are raised from said referencesurface.
 3. A system as claimed in claim 1, wherein said surfaceirregularities are depressed from said reference surface.
 4. A system asclaimed in claim 1, wherein said record comprises a transparentmaterial.
 5. A system as claimed in claim 1, further comprising auniform reflective coating abutting on said reference surface.
 6. Asystem as claimed in claim 1, wherein said record has the form of adisc.
 7. A system as claimed in claim 6, wherein said axis has the formof a spiral.
 8. A system as claimed in claim 6, wherein said trackportions are concentric circular track portions.
 9. A system as claimedin claim 1, wherein said record is made of a thermoplastic material;said surface irregularities being hot-pressed.
 10. A system according toclaim 1 wherein said photodetector assembly comprises two photodetectorelements arranged side by side in said detection plane; saidphotodetector elements receiving symetrically a portion of saidmodulated radiant energy; said photoelectric detection means furthercomprising electrical transmission means having two inputs respectivelyconnected to said photodetector elements, and an output delivering asignal proportional to the sum of the electrical signals respectivelysupplied by said photodetector element.
 11. A system as claimed in claim10, wherein said modulated radiant energy is collected by saidphotoelectric detection means upon being transmitted across said record.12. A system as claimed in claim 10, wherein said modulated radiantenergy is collected by said photoelectric detection means upon beingreflected by said phase modulating structure, a further reflection ofsaid modulated radiant energy being obtained from a semi-reflectiveplate arranged between said source and said objective lens.
 13. A systemas claimed in claim 10, wherein said photoelectric detection meansfurther comprise:a differential amplifier having two inputs respectivelyconnected to said photoelectric elements, and an output; a low passfilter having an input connected to the output of said differentialamplifier, and an electromechanical transducer controlled by saidlow-pass filter for imparting to said spot a displacement along adirection parallel to said reference surface, and at an angle with saidaxis.
 14. A system as claimed in claim 1, wherein said illuminationmeans comprise a source of radiant energy, and an objective lens forfocusing said radiant energy onto said phase modulating structure.
 15. Asystem as claimed in claim 14, wherein said source is a source ofcoherent radiant energy.